This invention relates to papermaking, and more particularly to a wet web transfer belt in a papermaking machine, wherein a wet web is transferred in a closed draw.
In a conventional papermaking machine wherein a wet web is transferred in open draw, that is, the wet web is transferred without a support, the web is susceptible to rupture at the transfer points. The possibility of rupture places a limitation on the speed at which the papermaking machine may be run. Consequently, closed draw papermaking machines, i.e. machines in which the web is transferred with a support, are becoming more prevalent. Closed draw machines can be operated stably at higher speeds.
In a conventional closed draw papermaking machine, a pickup felt picks up a wet web from a preceding section of the machine, holding the web underneath the bottom surface of the felt and transferring it to a next stage transfer belt.
The wet web is then pinched between the pickup felt and a transfer belt at a first nip formed between a pair of press rolls. Water is squeezed out of the wet web at the first nip. The wet web is then released from the pickup felt and transferred to the transfer belt. The transfer belt carries the wet web to a second nip formed between a third press roll and a shoe press, where further water is drained from the web and picked up by a press felt.
The wet web remains on the transfer belt until it reaches a vacuum roll, where the web is transferred to a canvas belt for heating and drying.
The closed draw papermaking apparatus utilizes a number of transfer points. Therefore, it is important for the wet web to be released easily to avoid maloperation. It is especially important for the wet web to be released easily at the location immediately following the shoe press because, if the transfer belt is smooth, a continuous film of water is likely to be formed between the wet web and the transfer belt. The water film can cause the wet web to adhere so tightly to the transfer belt that it cannot be released, even by the force applied by the vacuum roll.
The problem of web releasability has been addressed by utilizing the technology disclosed in Japanese published unexamined patent applications 57678/1994 and 88193/1985.
In the case of application 57678/1994 a filler is mixed into a synthetic resin layer formed on the surface of a base layer. The synthetic resin layer is ground after the resin has cured, and the filler forms projections on its surface. These projections impart a roughness to the surface and are very effective in breaking up the water film formed between the web and the transfer belt.
In the case of application 88193/1985, a synthetic resin layer 32 is applied to the surface of a base layer 31 by spraying in such a way that numerous air bubbles are formed in the synthetic resin layer. Upon grinding of the synthetic resin layer after it is cured, the air bubbles at the surface produce a multitude of concavities. These concavities impart a roughness to the surface of the belt which is effective to break the water film between the transfer belt and a wet web carried thereby.
The projections and concavities on the surfaces of the belts of the prior art can be formed only by grinding the surface of the synthetic resin layer after it has been cured. Especially in the case of the prior art belt of Japanese application 57678/1994, grinding must be carried out with a grinding tool operating against a rotating belt under a specific tension. In the grinding operation, there is a likelihood that fillers will be inadvertently scooped out, and consequently it is difficult to obtain a wet web transfer belt having the desired qualities.
Both of the above-mentioned prior art belts, which are composed of a base layer and a synthetic resin layer, tend to be heavy and difficult to install in a papermaking machine, depending on the structure and surroundings of the machine.
This invention is intended to solve the above-mentioned problems all at once. An important object of the invention is to provide a wet web transfer belt from which the wet web can be released easily at transfer points in a closed draw papermaking machine, and to provide a wet web transfer belt which can be easily installed.
The invention achieves the aforementioned objects in a wet web transfer belt for a papermaking machine wherein a wet web is transferred in a closed draw, the belt comprising a base layer and a batt layer, wherein the batt layer has a surface on which a wet web is placed, the surface being formed of fiber comprising a welded layer of meltable fiber, the welded layer including a filler which extends at least to said surface. The above belt structure makes it possible to obtain a desired surface roughness of superior quality, easily and reliably.
In a preferred embodiment, in order to prevent rewetting by keeping water from migrating deep into the belt, the welded layer has an air permeability of 2 cc/cm2/sec. or less.
The preferred surface roughness of the welded layer is in the range of Rz5-80 xcexcm in order that the water film formed between the wet web and the belt can be broken easily.
In accordance with a preferred embodiment of the invention, the hardness of the filler is higher than that of the welded layer, thereby enabling the filler to bite into the relatively soft welded layer of the meltable fiber, when compressed.
Other objects, details and advantages of the invention will be apparent from the following detailed description when read in conjunction with the drawings.